Why Cleavage in Chick is Called Discoidal Type?

In chick embryology, cleavage refers to the rapid division of the zygote into multiple cells. During cleavage, the cells divide without growth, resulting in a smaller size of each daughter cell.

In the chick, cleavage is referred to as discoidal type because it occurs within a small disc of yolk, known as the germinal disc, which is located at one pole of the egg. The yolk is a rich source of nutrients for the developing embryo, and the germinal disc contains the cytoplasm and nucleus of the egg.

Unlike other animals, where cleavage occurs throughout the entire zygote, the cleavage in the chick occurs only in the germinal disc, creating a disc-shaped mass of cells. This disc-shaped mass of cells will later develop into the embryo, while the yolk provides the necessary nutrients for the growing embryo.

Therefore, the cleavage in the chick is called discoidal type because it is limited to the germinal disc, which is a small disc-shaped area of the egg.